Speak with the vendor that is priintng the banner. Often they will ask for you to create your art work to scale (maybe 1:4 or 1:10). Their output devices are usually designed to print from smaller files. The line screen capabilities of a device change as the medium gets larger. Depending on your design, you could use either Photoshop or Illustrator and combine them in InDesign. If the art is all Vector (Illustrator), your artwork size is irrelevant. Vector art is infinitely scalable.